U.S. patent application number 11/762336 was filed with the patent office on 2007-12-27 for method of separating layers of material.
This patent application is currently assigned to J.P. SERCEL ASSOCIATES INC.. Invention is credited to Jongkook Park, Jeffrey P. Sercel, Patrick J. Sercel.
Application Number | 20070298587 11/762336 |
Document ID | / |
Family ID | 35061103 |
Filed Date | 2007-12-27 |
United States Patent
Application |
20070298587 |
Kind Code |
A1 |
Park; Jongkook ; et
al. |
December 27, 2007 |
METHOD OF SEPARATING LAYERS OF MATERIAL
Abstract
A lift off process is used to separate a layer of material from
a substrate by irradiating an interface between the layer of
material and the substrate. According to one exemplary process, the
layer is separated into a plurality of sections corresponding to
dies on the substrate and a homogeneous beam spot is shaped to
cover an integer number of the sections.
Inventors: |
Park; Jongkook; (Nashua,
NH) ; Sercel; Jeffrey P.; (Hollis, NH) ;
Sercel; Patrick J.; (Brentwood, NH) |
Correspondence
Address: |
GROSSMAN, TUCKER, PERREAULT & PFLEGER, PLLC
55 SOUTH COMMERICAL STREET
MANCHESTER
NH
03101
US
|
Assignee: |
J.P. SERCEL ASSOCIATES INC.
220 Hackett Hill Road
Manchester
NH
03102
|
Family ID: |
35061103 |
Appl. No.: |
11/762336 |
Filed: |
June 13, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11215248 |
Aug 30, 2005 |
7241667 |
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11762336 |
Jun 13, 2007 |
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11008589 |
Dec 9, 2004 |
7202141 |
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11215248 |
Aug 30, 2005 |
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60557450 |
Mar 29, 2004 |
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Current U.S.
Class: |
438/458 ;
257/E21.347; 257/E21.599 |
Current CPC
Class: |
B23K 26/0838 20130101;
B23K 2101/40 20180801; H01L 21/7806 20130101; B23K 26/57 20151001;
H01L 21/78 20130101; H01L 21/268 20130101; H01L 33/0093 20200501;
B23K 26/0732 20130101; H01L 2221/68363 20130101; B23K 26/53
20151001; H01L 2924/0002 20130101; B23K 2103/172 20180801; B23K
26/40 20130101; H01L 2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
438/458 ;
257/E21.347; 257/E21.599 |
International
Class: |
H01L 29/22 20060101
H01L029/22; H01L 21/30 20060101 H01L021/30 |
Claims
1. A method of separating at least one layer of material from a
substrate, the method comprising: providing first and second
substrates and at least one layer of material between the
substrates, the at least one layer of material being segregated
into a plurality of sections separated by streets; forming a beam
spot using a laser, wherein the beam spot is shaped to cover an
integer number of the sections; and irradiating an interface
between the first substrate and the sections using a step and
repeat process until the first substrate is separated from all of
the sections, wherein each repeated irradiation includes directing
the beam spot to cover at least one of the sections such that
stitching of the beam spot occurs within the streets between the
sections.
2. The method of claim 1 further comprising: moving the substrates
and the layer on a stage; and comparing a position of the stage
with a predetermined value, wherein the laser is triggered based on
the position to form the beam spot and irradiate the interface
between the first substrate and the at least one of the sections
covered by the beam spot.
3. The method of claim 1 wherein the sections are generally
rectangular and wherein the beam spot is generally rectangular.
4. The method of claim 1 wherein the substrate is a semiconductor
wafer, wherein the sections of the at least one layer correspond to
dies, and further comprising separating the dies after the
separating the first substrate.
5. The method of claim 1 wherein forming the beam spot includes
generating a raw laser beam, passing the raw laser beam through a
beam homogenizer to produce a homogenized beam, and cropping the
homogenized beam.
6. The method of claim 1 wherein the laser is an excimer laser, and
wherein irradiating the interface includes exposing the interface
to a single pulse of the excimer laser for each of the at least one
of the sections covered by the beam spot.
7. The method of claim 1 wherein irradiating is performed using a
laser energy density in a range of about 0.60 J/cm.sup.2 to 1.6
J/cm.sup.2.
8. The method of claim 1 further comprising: scribing the second
substrate on the streets between the sections after the first
substrate is removed; and separating the second substrate between
the sections.
9. The method of claim 1 wherein the first substrate includes a
sapphire wafer.
10. The method of claim 1 wherein the at least one layer includes
GaN.
11. The method of claim 1 wherein the second substrate includes
Molybdenum or its alloys.
12. The method of claim 1 wherein providing first and second
substrates and at least one layer of material between the
substrates comprises: providing the first substrate having the at
least one layer of material formed thereon; etching the at least
one layer of material to form the streets and the plurality of
sections; and attaching the second substrate to the sections.
13. The method of claim 12 wherein etching the at least one layer
includes using a laser beam to selectively remove portions of the
at least one layer of material in the streets.
14. The method of claim 13 further comprising applying a protective
coating to the at least one layer and removing the protective
coating after etching of the at least one layer and before
attaching the second substrate.
15. The method of claim 12 further comprising: forming a metal
substrate over the sections and the streets prior to attaching the
second substrate; cutting at least the metal substrate at locations
between the sections; and removing at least a portion of the metal
substrate after separating the first substrate from the
sections.
16. The method of claim 12 wherein the at least one layer includes
a GaN layer and at least one film including a reflective film
formed thereon, and wherein the second substrate includes
Molybdenum.
17. The method of claim 16 wherein the reflective film is an
aluminum film.
18. The method of claim 17 wherein the at least one layer further
includes a metallic film on the aluminum film, wherein the second
substrate including the Molybdenum attaches to the metallic
film.
19. The method of claim 1 wherein the substrate is a sapphire
wafer, and wherein the at least one layer of material includes at
least one layer of GaN.
20. The method of claim 19 wherein the at least one layer further
includes a GaN buffer layer.
21. The method of claim 20 wherein the laser light is generated
using a 248 nm excimer laser.
22. The method of claim 19 wherein the at least one layer further
includes an AlN buffer layer.
23. The method of claim 22 wherein the laser light is generated
using a 193 nm excimer laser.
24. A method of separating at least one layer of material from a
substrate, the method comprising: providing a first substrate
having at least one layer of material formed thereon; attaching a
second substrate to the at least one layer of material; forming a
beam spot using a laser; and irradiating an interface between the
first substrate and the layer of material by sequentially exposing
the interface to laser light at a plurality of different angles
within a range of angles with respect to the interface, to separate
the first substrate from the layer of material.
25. The method of claim 24 wherein irradiating is performed using a
laser energy density in a range of about 0.60 J/cm.sup.2 to 1.6
J/cm.sup.2.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of co-pending U.S. patent
application Ser. No. 11/215,248 filed Aug. 30, 2005, which is a
continuation of co-pending U.S. patent application Ser. No.
11/008,589 filed Dec. 9, 2004, now U.S. Pat. No. 7,202,141, which
claims the benefit of co-pending U.S. Provisional Patent
Application Ser. No. 60/557,450, filed on Mar. 29, 2004, which is
fully incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to separation of layers of
material and more particularly, to separation of layers of
material, such as a substrate and a film grown on the substrate, by
irradiating an interface between the layers.
BACKGROUND INFORMATION
[0003] GaN/InGaN-based Light-Emitting Diodes (LEDs), known as "Blue
LEDs," have a promising future. Practical applications for these
GaN/InGaN-based LEDs have been expanding to include such products
as mobile phone key-pads, LCD backlights, traffic lights,
commercial signs, automotive lights, outdoor full-color display
panels, household illuminative devices, and others. In these and
other applications, these high-brightness LEDs may replace
conventional light sources such as incandescent and fluorescent
lights. Blue LEDs are characterized by high light output at lower
energy input than conventional light sources (energy saving, high
efficiency) and a longer working life. Their high performance and
reliability shows promise for their successful replacement of
conventional light sources; however, there is a need to improve
current LED designs to overcome currently-known limitations and
inherent drawbacks. Better and more precise manufacturing
techniques help advance blue LED design by cutting waste,
increasing yields, and allowing more advanced and complex or
improved designs to emerge, advancing the technology through more
flexibility in Design for Manufacturability (DFM). Such improved
manufacturing techniques simplify and reduce the cost of their
manufacture.
[0004] Blue LED's may be fabricated by depositing GaN/InGaN
layer(s) on a sapphire substrate. Once the LED devices have been
fabricated, the wafer is separated into individual dies. One
current die separation process involves the following steps. First
the sapphire wafer is thinned to less than 100.mu.m in thickness by
grinding and lapping the backside of the wafer. Next the wafer is
mounted to dicing tape and then scribed along the streets between
the die by means of a diamond scribe tip or UV laser beam. Finally,
the wafer is fractured along the scribe lines by means of a
fracturing tool. After fracturing, the dicing tape is stretched so
as to physically separate the die from one another so that
subsequent automated pick and place operations can be performed.
This process is referred to as "scribe and break" die
separation.
[0005] A major cost of LED fabrication is the sapphire thinning and
the scribe-and-break operation. A process known as LED lift-off can
dramatically reduce the time and cost of the LED fabrication
process. LED lift-off may eliminate wafer scribing by enabling the
manufacturer to grow GaN LED film devices on the sapphire wafer,
for example, and then transfer the thin film device to a heat sink
electrical interconnect. In this process, the laser beam profile
fires through the back of a sapphire wafer to de-bond the GaN LED
device and transfer it to a substrate where it can then be packaged
onto a heat sink and/or optical reflector. Using special wafers,
the sapphire growth substrate may possibly be re-used, and the cost
of LED fabrication can be reduced. Additionally, this approach is
fast, delivering increased LED light output, and has low operating
costs due to low stress on the UV laser.
[0006] Current designs of GaN LEDs have inherent limitations that
hamper efforts to improve performance and reliability. The designs
have also been associated with electrostatic discharge problems. As
shown in FIGS. 1A and 1B, a blue LED 10 may include multiple InGaN
and GaN based layers 12a, 12b, 12c which are hetero-epitaxially
grown on a silicon carbide or a sapphire wafer substrate 14. Since
the sapphire wafer is a natural insulator, current is supplied by a
horizontal electrode configuration. Due to the high resistance of
the p-GaN layer 12a, a thin film of Ni/Au 16 is deposited over the
p-GaN to promote current dispersion spreading. However, there are
some drawbacks associated with the horizontal configuration.
[0007] First, the Ni/Au film 16 absorbs a substantial portion of
the LED light output. The Ni/Au film 16 is very thin (usually less
than 100 .ANG.), in order to make it transparent to LED light,
since it has limited transmittance to the emitting light.
Approximately 25% of the light emitted by the LED itself is
absorbed by the Ni/Au film 16. Furthermore, a significant
percentage of the emitted light is lost in transmission through the
sapphire. Some of light directed towards the sapphire substrate 14
is reflected to the front surface due to the difference in
refractive indices between the sapphire wafer and its surroundings.
The Ni/Au thin film 16 absorbs the majority of this reflected
output light as well.
[0008] Secondly, the Ni/Au film 16 is sensitive to moisture,
resulting in performance degradation over time. To maintain the
film's transparency, thin Ni/Au is deposited by metal evaporation,
and then heat-treated in an ambient air or an O.sub.2 environment.
The Ni/Au film 16 forms an oxidized compound, NiOx with an Au-rich
structure. When moisture penetrates to the oxide film over
long-term operation, the LED device 10 will be damaged.
[0009] Third, the Ni/Au film 16 experiences a degradation in the
performance efficiency of the InGaN MQW light-emitting layer 12b
due to a current crowding effect. Since the current spreading Ni/Au
film 16 has lower resistance than the n-GaN layer 12c, the current
may crowd in the region 18 near the n (-) electrode 20 (see FIG.
1A). Thus, the phenomenon of current crowding may prevent
homogeneous use of the active InGaN area, resulting in low
efficiency of light output and low reliability due to uneven use of
the active area.
[0010] Fourth, the horizontal-electrode configuration may create
the effect of a current bottleneck, resulting in low reliability.
The current supplied through the p (+) electrode 22 spreads across
the Ni/Au film 16, and flows from p-GaN 12a through InGaN 12b to
n-GaN 12c. Since the n (-) electrode 20 is horizontally located at
the n-GaN 12c, the current is bottlenecked in the area 24 at the
electrode 20 (FIGS. 1A and 1B).
[0011] A LED structured with a vertical electrode configuration
overcomes many of the drawbacks of the horizontal LED structure. As
shown in FIG. 2, an LED 30 with a vertical structure involves a
transfer of GaN layers 32a, 32b, 32c from the sapphire substrate to
a conductive substrate 34, such as a silicon wafer. The vertical
electrode configuration may eliminate the Ni/Au film, which
substantially increases light output. The vertical structure allows
the deposition of a metal reflection layer 36, which minimizes
light loss through the sapphire in the horizontal structure. The
vertical structure also improves reliability and performance by
reducing or eliminating the current crowding and bottle neck. A
factor in constructing the vertical LED structure is the successful
lift-off process of the GaN layer from the epitaxial sapphire wafer
to the conductive silicon wafer.
[0012] One example of the construction of a high-brightness
vertical LED is show in FIG. 3. First, GaN layers 32a, 32c are
deposited onto a sapphire wafer 38. After a metal thin-film
reflector 36 is deposited on the p-GaN, then a Si substrate, or any
other conductive substrate 34 (including GaAs substrate and thick
metal films) is bonded over the metal thin-film reflector. The
sapphire wafer is removed by UV-laser lift-off, as described below.
The n (-) electrode is deposited on the n-GaN layer and the p (+)
electrode is deposited on the Si wafer. Since the n-GaN layer has
lower resistance than the p-GaN layer, the thin Ni/Au film is no
longer needed. Current is therefore more evenly spread without
crowding or a bottleneck effect. Elimination of the troublesome
Ni/Au thin film results in an increase in performance and
reliability of LEDs with the vertical structure.
[0013] The vertical structure may be created using a UV-laser lift
off process. One approach to UV-laser lift-off involves the
selective irradiation of the GaN/Sapphire interface with a UV laser
pulse, utilizing the absorption difference of UV light between the
GaN (high absorption) thin film layers and the sapphire substrate.
Commonly, the GaN layers are hetero-epitaxially grown on a sapphire
wafer. To facilitate GaN crystal growth, a buffer layer may be
deposited at a relatively low temperature, around 300.degree. C.
While the buffer layer helps to grow the GaN layer at a high
temperature, the buffer layer contains a very high density of
various defects due to a large lattice mismatch. The crystal
defects, such as dislocations, nanopipes and inversion domains,
elevate surface energy which consequently increases absorption of
incident UV light. The incident laser beam for the lift-off process
carries an energy density well below the absorption threshold of
the sapphire wafer, allowing it to transmit through without
resulting in any damage. In contrast, the laser energy density is
high enough to cause photo-induced decomposition at the interface,
which allows debonding of the interface.
[0014] Studies exist regarding the UV laser lift-off process. Kelly
et al. demonstrated decomposition of GaN by laser irradiation
through transparent sapphire, using a Q-switched Nd:YAG laser at
355 nm. (see M. K. Kelly, O. Ambacher, B. Dalheimer, G. Groos, R.
Dimitrov, H. Angerer and M. Stutzmann, Applied Physics Letter, vol.
69 p. 1749, 1996). Wong et al. used a 248 nm excimer laser to
achieve separation of .about.5 .mu.m thin GaN film from a sapphire
wafer (see W. S. Wong, T. Sands and N. W. Cheung, Applied Physics
Letter, vol. 72 p. 599, 1997). Wong et al. further developed the
lift-off process on GaN LED using a 248 nm excimer laser (see W. S.
Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P. Bour, P. Mei, L. T.
Romano and N. M. Johnson, Applied Physics Letters, vol. 75 p. 1360,
1999). Kelly et al. also demonstrated the lift-off of 275 .mu.m
thick, free-standing GaN film using a raster scanning of Q-switched
355 nm Nd:YAG laser (see M. K. Kelly, R. P. Vaudo, V. M. Phanse, L.
Gorgens, O. Ambacher and M. Stutzmann, Japanese Journal of Applied
Physics, vol. 38 p. L217, 1999). Kelly et al. also reported their
difficulty in overcoming extensive fracturing of GaN thick film
upon the laser lift-off process, due to high residual stresses from
a GaN-sapphire wafer. Id. In this study, the authors had to heat
the GaN/sapphire wafer to 600.degree. C., but they could not
completely offset the fracturing problems caused by the residual
stresses.
[0015] In spite of the advantages from UV-laser lift-off, GaN LED
manufacturing has been limited due to poor productivity caused by
low process yield. The low yield is due in part to high residual
stresses in a GaN-sapphire wafer, resulting from a Metal-Organic
Chemical Vapor Deposit (MOCVD) process. The MOCVD process requires
an activation temperature of over 600.degree. C. As shown in FIG.
4A, GaN and InGaN layers 32 are deposited on a sapphire wafer 38 by
the MOCVD process. Since there is substantial difference in
coefficients of thermal expansion (CTE) between the GaN
(5.59.times.10-6/.degree. K) and the sapphire
(7.50.times.10-6/.degree.K) (see Table 1), high levels of residual
stresses exist when the GaN/sapphire wafer cools down to ambient
temperature from the high temperature of the MOCVD process, as
shown in FIG. 4B. The residual stresses include compressive
residual stresses 40 on the GaN and tensional residual stresses 42
on the sapphire. TABLE-US-00001 TABLE 1 Various material properties
of GaN and sapphire. Lattice Lattice Band Gap Thermal Const. a
Const. c Density Energy Expansion .times.10.sup.-6/ Material
Structure (.ANG.) (.ANG.) (g/cm.sup.3) (eV) .degree. K Sapphire
Hexagonal 4.758 12.991 3.97 9.9 7.50 GaN Hexagonal 3.189 5.815 6.1
3.3 5.59
[0016] When an incident laser pulse with sufficient energy hits a
GaN/sapphire interface, the irradiation results in instantaneous
debonding of the interface. Since the incident laser pulse has
limited size (usually far less than 1 cm.sup.2), it creates only a
small portion of the debonded or lifted-off interface. Since
surroundings of the debonded area still have high level of residual
stress, it creates a concentration of stress at the bonded/debonded
border, resulting in fractures at the border. This fracturing,
associated with the residual stress, has been one of the obstacles
of the UV-laser lift-off process.
[0017] Currently, there are different ways to perform laser
lift-off processes on GaN/sapphire wafers. One method involves
raster scanning of a Q-switched 355nm Nd:YAG laser (see, e.g., M.
K. Kelly, R. P. Vaudo, V. M. Phanse, L. Gorgens, O. Ambacher and M.
Stutzmann, Japanese Journal of Applied Physics, vol. 38 p. L217,
1999). This lift-off process using a solid state laser is
illustrated in FIG. 5A. Another method uses a 248 nm excimer laser
(see, e.g., W. S. Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P.
Bour, P. Mei, L. T. Romano and N. M. Johnson, Applied Physics
Letters, vol. 75 p. 1360, 1999). This lift-off process using an
excimer laser is illustrated in FIG. 5B.
[0018] Both processes employ raster scanning, as shown in FIG. 6,
which involves either translation of the laser beam 44 or the
target of the GaN/sapphire wafer 46. A problem associated with the
raster scanning method is that it requires overlapping exposures to
cover the desired area, resulting in multiple exposures 48 for
certain locations. In both of the above methods, the laser lift-off
of GaN/sapphire is a single pulse process. The unnecessary multiple
exposures in localized areas increase the potential for fracturing
by inducing excessive stresses on the film.
[0019] As shown in FIG. 7, raster scanning also involves a scanning
of the laser beam 44 from one end to the other, gradually
separating the GaN/sapphire interface from one side to the other.
This side-to-side relaxation of residual stresses causes large
differences in the stress level at the interface 50 between the
separated and un-separated regions, i.e., the interface between the
scanned and the un-scanned area. The disparity in residual stress
levels at the interface 50 increases the probability of propagation
of Mode I and Mode II cracks. Although the illustrations in FIGS. 6
and 7 are based on a process using a solid state laser, raster
scanning of an excimer laser will produce similar results.
[0020] Currently, a common size of sapphire wafers is two-inch
diameter, but other sizes (e.g., three-inch and four-inch wafers)
are also available for the hetero-epitaxial growth of GaN. For a
GaN/sapphire wafer, the level of residual stresses varies in the
wafer, and compressive and tensile residual stresses may exist
together. The existence of the residual stresses may be observed by
wafer warping or bowing. When a laser lift-off process relaxes a
large area of a continuous GaN/sapphire interface, as described
above, a severe strain gradient may be developed at the border
between the debonded and the bonded interface. This strain gradient
may cause extensive fracturing of the GaN layer.
[0021] When a target material is irradiated with an intense laser
pulse, a shallow layer of the target material may be
instantaneously vaporized into the high temperature and high
pressure surface plasma. This phenomenon is called ablation. The
plasma created by the ablation subsequently expands to
surroundings. The expansion of the surface plasma may induce shock
waves, which transfer impulses to the target material. The ablation
may be confined in between two materials when the laser is directed
through a transparent material placed over the target. During this
confined ablation, the plasma trapped at the interface may create a
larger magnitude of shock waves, enhancing impact pressures. The
explosive shock waves from the confined ablation at the
GaN/sapphire interface can cause not only separation of the GaN
layer from the sapphire substrate but may also fracture the GaN
layer near the laser beam spot (see, e.g., P. Peyre et. al.,
Journal of Laser Applications, vol. 8 pp.135-141, 1996).
[0022] Accordingly, there is a need for an improved method of
separating GaN thin films from a sapphire wafer by addressing the
problems associated with residual stress, which lead to low yields
due to the fracturing of separated film layers. There is also a
need for processes that can be extended to any lift-off
applications to address one or more of the problems discussed
above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] These and other features and advantages will be better
understood by reading the following detailed description, taken
together with the drawings wherein:
[0024] FIG. 1A is a schematic diagram illustrating a cross section
of a conventional GaN LED with a horizontal electrode
configuration.
[0025] FIG. 1B is a top view of the GaN LED shown in FIG. 1A.
[0026] FIG. 2 is a schematic diagram illustrating a cross section
of a GaN LED with a vertical electrode configuration.
[0027] FIG. 3 is a flow diagram illustrating construction of a GaN
LED with a vertical electrode configuration.
[0028] FIG. 4A is a schematic diagram illustrating a GaN/sapphire
wafer during a MOCVD process.
[0029] FIG. 4B is a schematic diagram illustrating formation of
residual stresses on a GaN/sapphire wafer after a MOCVD
process.
[0030] FIG. 5A is a schematic diagram illustrating a conventional
method of laser lift-off on a GaN/sapphire wafer using a Q-switched
355 nm Nd:YAG laser.
[0031] FIG. 5B is a schematic diagram illustrating a conventional
method of laser lift-off on a GaN/sapphire wafer using a 248 nm
excimer laser.
[0032] FIG. 6 is a schematic diagram illustrating raster scanning
of a Q-switched 355 nm Nd:YAG laser on a GaN/sapphire LED wafer and
the resulting multiple exposures.
[0033] FIG. 7 is a schematic diagram illustrating raster scanning
on a GaN/sapphire LED wafer and the resulting stresses, which
create a high probability of Mode I and II cracks at the
interface.
[0034] FIG. 8 is a schematic diagram of the use of a laser pulse to
induce a shock wave for separating layers, consistent with one
embodiment of the present invention.
[0035] FIG. 9 is a schematic diagram illustrating a laser exposed
area and cross-section of the separation of the layers, consistent
with one embodiment of the present invention.
[0036] FIGS. 10A-10C are schematic diagrams illustrating the
effects of different laser energy densities.
[0037] FIG. 11 is a schematic diagram of a wafer illustrating
selective ablation of GaN layers on streets to separate the GaN
layers into a plurality of dies, leaving the sapphire wafer intact,
consistent with one embodiment of the present invention.
[0038] FIG. 12 is a schematic diagram of a beam delivery system
illustrating the projection of a homogeneous beam and
representative beam profile shown along the beam path, consistent
with another embodiment of the present invention.
[0039] FIG. 13 is a schematic diagram of a wafer illustrating laser
lift-off exposure using a step and repeat process, consistent with
a further embodiment of the present invention.
[0040] FIG. 14 is a photograph of a wafer illustrating a single
pulse exposure on a three-by-three LED array using the step and
repeat lift-off process.
[0041] FIG. 15 is a diagram illustrating a laser lift-off process,
combining the segregation of residual stress and precision
step-and-repeat laser beam exposure, consistent with yet another
embodiment of the present invention.
[0042] FIG. 16 is a photograph of a wafer illustrating selective
removal of GaN by a solid state UV laser with a variable astigmatic
focal beam spot.
[0043] FIG. 17 is a schematic diagram illustrating concentric or
helical laser lift-off exposure with a square beam, consistent with
a further embodiment of the present invention.
[0044] FIG. 18 is a schematic diagram illustrating concentric or
helical laser lift-off exposure with a circular beam, consistent
with a further embodiment of the present invention.
[0045] FIG. 19 is a schematic diagram illustrating concentric laser
lift-off exposure with a variable annular beam, consistent with a
further embodiment of the present invention.
[0046] FIG. 20 is a diagram illustrating a laser lift-off process,
consistent with yet a further embodiment of the present
invention.
[0047] FIG. 21 is a side schematic view of a lift-off exposure
applied to an interface between a substrate and layer(s) at a range
of angles, consistent with a further embodiment of the present
invention.
DETAILED DESCRIPTION
[0048] This detailed description describes exemplary embodiments of
processes consistent with the present invention, which address the
problems associated with existing lift-off processes and increase
productivity. Applications of the invention are not limited to the
following exemplary embodiments. Although the exemplary embodiments
refer to GaN and sapphire and the GaN/sapphire interface, other
types of substrates and layers of material may be used which are
known to those skilled in the art. Also, a sacrificial layer can be
provided between the GaN (or other layer of material) and the
sapphire (or other type of substrate).
[0049] Referring to FIG. 8, a laser may be directed through at
least one layer of substrate material 102 to at least one target
material 104 to separate the materials 102, 104. In the exemplary
embodiment, the substrate material 102 is sapphire and the target
material 104 is gallium nitride (GaN). The separation of the
materials 102, 104 may be achieved by using a laser energy density
sufficient to induce a shock wave at the interface 106 of the
target material 104 and the substrate material 102, thereby
instantaneously debonding the target material 104 from the
substrate material 102. The shock wave may be created by the
explosive expansion of plasma 108 at the interface as a result of
the increased density of the ionized vapor sharply elevating the
plasma temperature. The laser energy density may be in a range
sufficient to induce a force F.sub.a on the target material 104
that causes separation without fracturing. The applied force
F.sub.a may be represented as follows: [0050]
P.sub.p(GPa)=C[I.sub.r(GW/cm.sup.2)].sup.1/2 [0051]
F.sub.a(N)=P.sub.p(GPa)A.sub.r(cm.sup.2) where P.sub.p is the peak
pressure induced by explosive shock waves, C is an efficiency and
geometrical factor, I.sub.r is the irradiance of the incident laser
beam, F.sub.a is the applied force and A, is the area under
irradiation.
[0052] When the plasma 108 is expanding, as shown in FIG. 9, the
laser exposed area is acting as a bending arm pivoting at the edge
of the laser exposed area. For example, the force (F.sub.r)
required for rupturing or fracturing may be viewed as a two-point
bend test and may be represented as follows: F r .varies. wd 2 L
.times. .sigma. r ##EQU1## where d is the thickness of the target
material 104, w is the width of the applied force or width of the
laser pulse, L is the length of applied arm or half length of the
laser pulse, and .sigma..sub.r is the modulus of rupture or
fracture stress of GaN. To increase the force (F.sub.r), the width
w of the laser pulse may be increased and the half length L of the
laser pulse may be decreased, thereby forming a line shaped beam.
The line shaped beam may be scanned across the target material 104
to minimize the bending moment upon irradiation.
[0053] At a laser energy density under the ablation threshold of
GaN (.about.0.3 J/cm.sup.2 at 248 nm), for example, the
instantaneous separation of the GaN/sapphire interface 106 may not
be successfully achieved, as shown in FIG. 10A. Although
decomposition of the GaN can occur under the ablation threshold,
this alone cannot achieve instantaneous separation of the interface
106, because there is no driving force, i.e. shock waves from the
expanding plasma, without the ablation. Conversely, applying
overly-intense laser energy density may create excessive explosive
stress wave propagation, which results in cracks and fractures on
the target material 104 (e.g., the GaN film), as shown in FIG. 10C.
When the irradiating laser energy density is optimized, as shown in
FIG. 10B, the force created by the shock wave is sufficient to
separate the layers 102, 104 at the interface 106 but not enough to
induce fracture in the target material 104. According to one
exemplary embodiment with GaN and sapphire, the optimum range of
laser energy density may be between about 0.60 J/cm.sup.2 to 1.5
J/cm.sup.2.
[0054] The parameters of the laser irradiation, such as the
wavelength and energy density, depend on the types of materials
being separated. For example, the optimum laser energy density for
separating GaN from sapphire is discussed above. A laser wavelength
of 248 nm is also desirable for separating GaN from sapphire. It is
well known to those skilled in the art that the photonic energy of
248 nm (5 eV) is between the bandgaps of GaN (3.4 eV) and sapphire
(9.9 eV). This indicates that the 248 nm radiation is better
absorbed in GaN than in sapphire and the selective absorption
causes the ablation resulting in separation.
[0055] Those skilled in the art will recognize that other laser
wavelengths may be used to separate other types of materials. For
example, a buffer layer may be used between the sapphire substrate
and the GaN layer(s) to facilitate epitaxial growth of the GaN.
Examples of the buffer layer include a GaN buffer layer and an
Aluminum Nitride (AlN) buffer layer. Where an AlN buffer layer is
used, a laser at 193 nm may be used because the photonic energy of
the 193 nm laser light (6.4 eV) is in between bandgaps of sapphire
(9.9 eV) and AlN (6.1 eV).
[0056] According to one embodiment of the present invention, as
shown in FIG. 11, one or more of the layers to be separated (e.g.,
the GaN film or layers) may be formed into smaller areas or
sections 112 before lift-off or separation from a substrate 110
such as a sapphire wafer. In one embodiment, the sections 112 may
be segregated, for example, to correspond to LED dies. The
formation of sections 112 reduces fractures induced by residual
stresses and shock waves at the interface during a lift-off
process. The sections 112 of GaN film are less influenced by
induced residual stresses from its surroundings. Furthermore, the
sections 112 have an insignificant amount of residual stresses and
strains, which the thin GaN film in these sections 112 can
withstand.
[0057] According to one example, a GaN/sapphire LED wafer 116
contains symmetric and repeating patterns of sections 112 to form
the same-sized LED dies, which are generally in a few hundreds of
microns of square or rectangular size. The symmetrical and
repeating sections 112 may be separated by streets 114, for
example, which determine borders for the LED dies and provide
sacrificial spaces for die separation, for example, using a scribe
and break process. Although the sections 112 in the exemplary
embodiment correspond to individual square-shaped dies, those
skilled in the art will recognize that other configurations and
shapes, such as rectangular shapes, may be formed.
[0058] The GaN film can be separated into sections 112 through
selectively removing or etching of GaN layer(s) on the streets 114.
One method of selectively removing GaN layer(s) on the streets 114
is through reactive ion etching, which is generally known to those
skilled in the art. This process has a few drawbacks, including
slow etch rate and the requisite handling of hazardous chemicals.
Another method includes the selective etching by a solid state UV
laser with a variable astigmatic focal beam spot formed by an
anamorphic beam delivery system, as disclosed in U.S. patent
application Ser. No. 10/782,741, which is fully incorporated herein
by reference. The variable astigmatic focal beam spot can
effectively adjust its size to an optimum laser energy density,
which selectively ablates GaN layer(s) on the streets 114, leaving
the sapphire substrate unaffected (see FIG. 11). This selective GaN
ablation utilizes the large difference in ablation threshold
between GaN (0.3 J/cm.sup.2 at 248 nm) and sapphire (over 2
J/cm.sup.2 at 248 nm).
[0059] According to another method, the etching can be performed
using a patterned laser projection (e.g., using an excimer laser).
A patterned excimer laser beam can also be used to dry pattern the
GaN streets or the devices into shapes or to pattern other thin
films such as ITO, metallization, or dielectric insulation layers,
or for other devices or conductive or insulator layers. As an
alternative to removing portions of a continuous GaN film to form
the sections 112 and streets 114, the GaN can be formed (e.g.,
grown) on the substrate 110 as sections 112 and streets 114. The
growth of the continuous GaN film, however, may be more economical
as compared to the growth of GaN layers with patterns of streets
114 and sections 112.
[0060] According to a further method, the streets 114 between the
sections 112 may be widened, for example, using reactive ion
etching, after the substrate 110 has been removed. Re-etching the
streets 114 may reduce or eliminate the possibility of current
leakage at the side walls of the sections 112, for example, at the
n-GaN and p-GaN junction.
[0061] A lift-off process may be used to separate the sections 112
(e.g., the GaN layer(s)) from the substrate 110 (e.g., the sapphire
wafer) by irradiating an interface between the substrate 110 and
the sections 112. The exemplary laser lift-off process may use a
single-pulse process with a homogeneous beam spot and an energy
density sufficient to induce a shock wave as described above. The
single pulse process avoids overlapping exposures at the interface
between the substrate 110 and the sections 112 and thus minimizes
fracturing. The homogeneous beam spot may be used to irradiate the
interface between layers being separated to substantially eliminate
the density gradient, thereby facilitating effective lift-off. Both
UV solid state lasers and excimer lasers can be used with a beam
homogenizer to generate a homogenous beam spot for the lift-off
process. One exemplary embodiment uses a KrF excimer laser at 248
nm. The gaseous laser medium with electrical discharge generates
high average power with a large raw beam size. The application of
the beam homogenizer is very effective with the large and powerful
raw beam of the excimer laser. Also, providing an
evenly-distributed laser energy density in a beam spot
advantageously creates effective lift-off in the area with the
single pulse irradiation.
[0062] FIG. 12 illustrates one example of a projection of a
homogeneous beam by near-field imaging and shows a representative
beam profile along the beam path. The raw beam from an excimer
laser 120 has Gaussian distribution in short sided/flat topped
distribution in the long side. The beam homogenizer 122 (e.g., of
multi-array configuration) makes the gradient raw beam profile into
a square flat-topped profile. The homogenized beam is cropped by
the mask 124 (e.g., the rectangular variable aperture) to utilize
the best portion of the beam, which is projected to the LED target
wafer 116 by near-field imaging, for example, using beam imaging
lens 126. The edge resolution of the homogeneous beam spot 130 at
the LED wafer 116 therefore becomes sharp. Although one
configuration for the beam delivery system is shown in this
exemplary embodiment, those skilled in the art will recognize that
other configurations may be used to create and project the
homogeneous beam. Although the exemplary embodiment shows a mask
124 with a rectangular aperture, any shape of mask can be used for
near field imaging.
[0063] According to one exemplary method, the lift-off exposure is
performed using a step and repeat process. The homogenized beam
spot 130 is shaped to include an integer number of segregated
sections 112 (e.g., corresponding to an integer number of LED
dies), as shown in FIG. 13. The size of the beam spot 130 is
precisely shaped, based on the size of segregated sections 112, to
include multiple segregated sections 112, such as a three-by-three
array. The interface between the substrate and the integer number
of sections 112 may be irradiated using a single pulse exposure,
and the process may be repeated for each group of sections (i.e.,
dies). The numbering in FIG. 13 shows an exemplary sequence for the
step and repeat process. As the irradiation is repeated for each
group of sections 112, stitching of the beam spot 130 may be
performed. Advantageously, the stitching may be performed within
the streets 114 to avoid possible damages in the active LED area.
In the exemplary process, the stitching of the beam spot 130 is
kept within about 5 .mu.m
[0064] FIG. 14 illustrates an example of a single-pulse exposure on
an LED lift-off wafer by 248 nm excimer laser. In FIG. 14, the
homogenized beam spot is covering nine (9) LED dies and the
debonded GaN/sapphire interface appears brighter.
[0065] Due to its precisely controlled exposure with a single pulse
in a small area, the exemplary laser lift-off exposure does not
require heating of the LED wafer to offset the residual stresses.
Exposure may be performed at room temperature. Because the laser
light of the lift-off exposure travels through the sapphire wafer,
damage or debris on the surface of the sapphire can make shadows at
the GaN/sapphire interface, causing defects on the lift-off
interface. The surface of the sapphire may be polished to remove
any debris or particles. The lift-off exposure may also be applied
to the target in a range of different angles, which will reduce or
eliminate shadowing effects.
[0066] The exemplary processes discussed above can improve the
productivity and yield of the UV-laser lift-off process for
successful industrial applications. An exemplary method consistent
with the present invention combines the segregation of residual
stress on a LED wafer and the homogenous beam laser exposure. The
selective etching of GaN layers on streets isolates the film into
small areas, which have minimal influence by residual stresses from
its surroundings. In addition, the small areas themselves have
minimal residual stresses, which will hardly affect the GaN film
upon lift-off exposure. The homogenized beam delivers substantially
uniform laser energy density in the spot. The precise laser
exposure with a homogenized laser beam allows the proper lift-off
with optimum laser energy density.
[0067] FIG. 15 illustrates an exemplary lift-off process. After one
or more GaN layer(s) 132 are grown on a sapphire substrate 110, a
protection coating 135 can be applied to prevent deposition of
laser generated debris on the GaN layer(s) 132 in the wake of laser
scribing. The selective removal of the GaN layer(s) 132 to form
streets 114 and sections 112 can be done by laser scribing or by
reactive ion etching. A conductive substrate 134 is bonded on the
GaN layers 132, after the protection coating 135 is removed. The
conductive substrate 134 can be any type of conductive ceramics and
metals, including but not limited to, Si, Ge, GaAs, GaP, Copper,
Copper Tungsten and Molybdenum. A reflective layer (not shown) may
also be formed between the GaN layer(s) 132 and the conductive
substrate 134. Then, the sapphire substrate 110 can be removed by
the laser lift-off process. After the laser lift-off, the GaN
surface can be treated for deposition of electrode metal film or
other necessary steps. Finally, the wafer can be separated between
the sections 112, for example, to form individual LED dies.
[0068] An example of the selective removal of the GaN layer on an
actual wafer is shown in FIG. 16, where a solid state UV laser
providing a high speed laser cut with a variable astigmatic focal
beam spot was used for the GaN removal. In this example, monolithic
GaN layers, which contain no die or street pattern, were grown on a
sapphire wafer initially. The LED die size is defined by the lines
cut by the laser. In the example, the width of selective removal or
laser cuts is only about 5 .mu.m, which minimizes the loss of the
wafer real estate.
[0069] Among the conductive substrates for the lift-off, Molybdenum
has desirable properties, such as matching coefficient of thermal
expansion (CTE), high reflectivity in blue spectra and high
strength with low ductility. Molybdenum has a CTE
(4.8.times.10.sup.-6/K) that is relatively close to that of GaN
(5.6.times.10.sup.-6/K). Metal compounds, such as PdIn and SnAu,
may be used for the bonding of the conductive substrates on GaN.
When using these bonding materials, the GaN and the substrate is
heated, for example, up to around 400.degree. C. A large mismatch
of CTE between the GaN layers and the lift-off substrate may
introduce another high level of residual stresses, which are
detrimental to the bonding process. For example, although Cu has
great thermal and electrical conductivity, it is not as desirable
as a lift-off substrate with a 2 inch GaN/sapphire wafer because of
its high CTE (16.5'10.sup.-6/K).
[0070] Molybdenum has reflectivity of about 55% in the blue
spectral region, ranging from 350 nm to 450 nm. This value is
comparable to other metals. For instance, the reflectivity values
of major metals at 410 nm are as follows: gold (37%), copper (51%),
Nickel (50%), platinum (57%), iron (58%), chromium (70%), silver
(86%), aluminum (92%). Although the comparable reflectivity allows
molybdenum to be directly used as a reflector (i.e., without a
separate reflective layer), the light output can be maximized by
deposition of a metal film with high reflectivity, such as aluminum
and silver. A highly reflective film layer between GaN and
molybdenum can increase the performance of a blue LED, for example,
without introducing a high level of residual stresses. For example,
aluminum can be deposited by sputtering on the GaN surface to form
the reflective layer. Since the oxidation of aluminum film is
detrimental to the bonding to the molybdenum substrate, another
layer of metallic film can be deposited to prevent the oxidation
and enhance the bonding. Examples of metallic films that do not
oxidize and that will allow the molybdenum to adhere to the
aluminum film include, but are not limited to, tin, zinc, lead and
gold.
[0071] Molybdenum also provides advantages during the die
separation process. Conventional diamond saw or diamond scribing
are difficult to use for separation of a metal film, mainly due to
its high ductility. Laser cutting and scribing is an alternative
method for die separation. However, a metal film with high
ductility, such as copper, requires 100% through cut for the
separation, because the mechanical breaking is difficult on ductile
substrates. Thus, the laser through-cut raises handling issues
because it may not maintain the integrity of small dies after the
cut. Molybdenum has high strength and low ductility. These unique
mechanical properties of molybdenum facilitate the mechanical
breaking, even when it is laser-scribed for about 90% of its
thickness.
[0072] According to another exemplary method, a laser lift-off
exposure may be combined with a technique of high speed motion
control to maximize productivity. When the laser lift-off utilizes
the step-and-repeat exposure with precisely designed beam
stitching, it is desirable for the triggering of the laser to be
accurate on the target. The fastest possible speed of the
step-and-repeat process is also desirable to increase productivity.
A special function of motion control can be used to compare the
position of the motion stages and send a trigger signal to the
laser at predetermined positions. The technique is referred to as
`position compare and trigger` or `fire on fly.` While motion
stages are in continuous motion, a processor in a motion controller
is constantly comparing an encoder counter to user programmed
values, and sending out trigger signals to a laser with matching
values. Thus, the motion stages do not need to stop for the step
and repeat, but may move in continuous motion, i.e., the laser
fires on fly. For example, when the lift-off process utilizes a
fire on fly technique, the homogeneous beam spot size of 1.times.1
mm.sup.2, with pulse repetition rate of a laser at 200 Hz, can
perform the lift-off process of 2 inch diameter LED wafer within
about a minute.
[0073] Although the exemplary embodiments involve forming the
streets 114 and sections 112 before performing the lift-off
process, the techniques described herein may also be used to
separate continuous layers without first segregating a layer into
sections. Although effective separation of continuous layers is
possible, there may be micro-cracks formed where the laser pulses
overlap.
[0074] Other exemplary methods may use unique techniques to scan
the laser beam for the lift-off exposure, for example, to irradiate
in a concentric pattern. These techniques may be used to perform
lift-off of one or more segregated layers or one or more continuous
layers on a substrate. The residual stresses in a GaN/sapphire LED
wafer have a concentric distribution, where tension and compression
exist together. The laser exposure, when crossing the wafer center,
may cause large differences in the stress level at the interface
between the separated and un-separated regions, i.e., the interface
between the scanned and the un-scanned area. According to different
methods, the beam may be scanned with a circular, spiral or helical
exposure to relax the residual stresses along with locations at the
same level of stresses. This method reduces the stress gradient at
the interface between scanned and unscanned areas. Alternatively, a
line beam with dimensions to minimize bending moment upon
irradiation may be scanned across the interface, as discussed
above.
[0075] FIG. 17 shows a concentric lift-off exposure with a square
beam spot 150. FIG. 18 shows a concentric lift-off exposure with a
circular beam 152. In one method, the laser beam is stationary
while the wafer is translated concentrically (e.g., in a circular
or helical pattern) for the exposure. According to another method,
the beam may be moved (e.g., in a circular or helical pattern) on a
stationary wafer.
[0076] One way to move the circular beam is using galvanometer
scanners, which precisely control two mirrors in motion by rotary
motors. Other beam spot shapes known to those skilled in the art
may also be used, such as triangular, hexagonal or other polygon
shapes. In the case of a polygon shaped laser pattern irradiating a
polygon shaped die, the beam may be moved in a circular or spiral
motion to overlay the die or groups of die and provide separation
of the film from the substrate in a controlled pattern to relieve
stresses in a controlled way.
[0077] Another alternative achieves the concentric scanning using a
variable annular beam spot. As shown in FIG. 19, the variable
annular beam spot 154 gradually reduces its diameter to
concentrically scan from outer edges to the center of the wafer.
The variable annular beam spot can be achieved by an incorporation
of two conical optics into the beam delivery system (BDS), where
the distance between the two optics determines the diameter of the
spot. Using this annular beam spot moving concentrically provides
stable relaxation of the residual stresses upon the laser lift-off
exposure.
[0078] FIG. 20 illustrates a laser lift off process for separating
an electroplated substrate, consistent with a further embodiment. A
sapphire wafer or substrate 110 with sections 112 of GaN formed
thereon may be electroplated with a metal or metal alloy to form a
metal substrate 160. Nickel or copper, or alloys thereof, may be
used for electroplating. The metal substrate 160 may then be cut at
locations 162 between the sections 112, for example, using a UV
laser. A supporting film 164 may be mounted on the metal substrate
160, and a laser lift-off process such as described above may then
be used to separate the sapphire substrate 110. Post laser lift-off
processes such as contact metallization may then be used to remove
portions of the metal substrate 160 to form dies 166. The dies 166
may then be separated. By cutting the metal substrate 160 prior to
laser lift-off, the integrity of the dies 166 can be maintained
because of the bonding to the sapphire substrate 110. Those skilled
in the art will recognize that this process may also be performed
using other materials.
[0079] In summary, according to a method consistent with one aspect
of the present invention, first and second substrates are provided
with at least one layer of material between the substrates, the
layer of material being segregated into a plurality of sections
separated by streets. A beam spot is formed using a laser and
shaped to cover an integer number of the sections. An interface
between the first substrate and the sections is irradiated using
the beam spot. The irradiating is performed repeatedly for each
integer number of the sections until the first substrate is
separated from all of the sections.
[0080] According to another method, a substrate is provided having
at least one layer of material formed thereon and a homogenous beam
spot is formed using at least a laser and a beam homogenizer. An
interface between the layer and the substrate is irradiated with
substantially evenly-distributed laser energy density using a
single pulse of the homogeneous beam spot to separate the layer
from the substrate.
[0081] According to yet another method, a substrate is provided
having at least one layer of material formed thereon and a beam
spot is formed using a laser. An interface between the first
substrate and the layer is irradiated using the beam spot. The
interface is irradiated in a generally concentric pattern to
separate the layer from the substrate.
[0082] According to a further method, a first substrate is provided
having at least one layer of material formed thereon and the
layer(s) of material are etched to segregate the layer(s) into a
plurality of sections separated by streets on the first substrate.
A second substrate is attached to the sections and a homogenous
beam spot is formed using a laser. The homogeneous beam spot is
shaped to cover an integer number of the sections. An interface
between the first substrate and the sections is irradiated using
the homogeneous beam spot. The irradiating is performed repeatedly
for each integer number of the sections. The first substrate is
separated from all of the sections.
[0083] According to yet another method, a first substrate is
provided having at least one layer of GaN formed thereon and at
least one film is formed on the GaN layer. The film may include a
reflective film. A second substrate including Molybdenum is
attached to the film and an interface between the first substrate
and the GaN layer is irradiated to separate the first substrate
from the layer of GaN.
[0084] While the principles of the invention have been described
herein, it is to be understood by those skilled in the art that
this description is made only by way of example and not as a
limitation as to the scope of the invention. Other embodiments are
contemplated within the scope of the present invention in addition
to the exemplary embodiments shown and described herein.
Modifications and substitutions by one of ordinary skill in the art
considered to be within the scope of the present invention, which
is not to be limited except by the following claims.
* * * * *